Base station with time-aligned mobile transmitting moments according to distance to base station

ABSTRACT

For increasing the radius of the effective service area of a base station in a digital time division multiple access (TDMA) radio network, a first transceiver is provided for communicating with mobile radio stations only within a radially outer annular region, and a second transceiver is provided for communicating with the mobile radio stations within a central region a radially outer marginal portion of which overlaps with a radially inner marginal portion of the outer annular region. Timing offsets between transmissions from the base station transceivers and transmissions from the mobile stations being such that respective transmissions are received within proper time slots despite that the outer radius of radially outer annular region would necessitate a larger than workable time delay were the first transceiver used by itself for serving the entire radial extent of the respective composite effective coverage areas of the first and second transceivers.

This application claims benefit of international applicationPCT/FI94/002989 filed Jun. 29, 1994.

BACKGROUND OF THE INVENTION

The invention relates to a base station for a digital time divisionmultiple access radio network, the base station comprising a radiotransceiver, means for measuring a time difference between thetransmissions of the transceiver and a mobile radio station due to adistance between the base station and the mobile radio station, andmeans for controlling the mobile radio station to adjust itstransmission relative to the transmission of the transceiver by apredetermined timing advance compensating for said time difference, saidtiming advance having a maximum value determining a maximum radiusr_(max) for an effective service area of the base station transceiver.

In digital Time Division Multiple Access (TDMA) radio systems, aplurality of mobile radio stations may utilize the same radio channel ona time division basis for communication with the base station.Communication on the radio channel takes place in a number of successivetime slots, e.g. 8 time slots, allocated to the users as required. Amobile radio station is synchronized with a signal from the basestation, and it transmits in accordance with this sync so that thesignal from the mobile station is received at the base station in a timeslot allocated for this particular mobile station. Mobile stations,however, may be located at different distances from the base station,whereby the propagation delay caused by this distance has to be takeninto account in the synchronization of the timing of transmission ofeach mobile station with the base station so that the signal will bereceived at the base station in the right time slot. For this purpose,the base station measures the time difference between its owntransmission and the transmission received from the mobile station, onthe basis of which it defines a suitable timing advance for the mobilestation. The mobile station uses this timing advance to advance thetiming of its transmission relative to a basic time instant defined bythe sync received from the base station. Various intrasystem featureslimit the timing advance to a certain maximum. This maximum of thetiming advance, in turn, dictates the maximum cell size that the basestation of the system is able to serve. In the Pan-European mobile radiosystem GSM (Global System for Mobile Communication), for instance, thetiming advance may vary in the range between 0 and 233 μs, which means acell size with a maximum radius of 35 km. However, such a cell size of35 km may be too small e.g. in sparsely populated areas with a lowtraffic load. Due to the above-mentioned limitation, however, increasingthe cell size has not been possible even though the capacity of the basestation and the topology of the terrain would otherwise have allowedlarger cells.

SUMMARY OF THE INVENTION

The object of the present invention is to dispense with the aboveproblem.

This is achieved by a base station of the type described in theforegoing BACKGROUND section, which according to the invention ischaracterized by means for providing a timing offset between thereception and the transmission of the transceiver in a manner that themaximum radius of the service area of the transceiver is r₁ +r_(MAX),and the service area has a central area which has a radius r₁ and withinwhich the transceiver is not capable of providing services to mobileradio stations, where r₁ is directly proportional to the timing offset;a second radio transceiver where the timing between reception andtransmission is normal and the outer radius of the service area of whichis at a distance r_(MAX) from the base station; the offset betweentransmission and reception being selected so that the service areas ofthe first-mentioned transceiver and the second transceiver overlap.

In the invention, an extended coverage area is provided around the basestation by offsetting the timing of the transceiver of the base stationas compared with the timing of the transmitter. The transceiveroperating in this way forms an extended, typically ring-shaped servicearea around the base station. Within the service area, a central shadowarea remains, the maximum radius of which is directly proportional tothe above-mentioned offset. The maximum extended outer radius of theservice area is the sum of the maximum radius defined by the maximumtiming advance and the maximum radius of the central shadow area.Accordingly, in the GSM system, for instance, if the offset is selectedsuch that the inner radius of the ring-shaped service area is 20 km, itsouter radius will be 60 km. The base station according to the inventionalso has a normal transceiver with a normal service area of 0 to 35 km,these service areas together form a cell having a radius of 60 km. Thesetwo service areas overlap to some extent in order that the intra-basestation handover between the two service areas could be performedunnoticed. In the preferred embodiment of the invention, the timingadvance assigned to the mobile station is used as a handover criterion.

DESCRIPTION OF THE INVENTION

In the following the invention will be described more fully by means ofillustrating embodiments with reference to the attached drawings, inwhich:

FIG. 1 is a block diagram illustrating a base station according to theinvention,

FIG. 2 illustrates the timing of the transmitters and receivers of thebase station shown in FIG. 1,

FIG. 3 illustrates an extended cell according to the invention, and

FIG. 4 illustrates another extended cell according to the invention.

DETAILED DESCRIPTION

The present invention is intended to be applied in any radio networkutilizing digital time division multiple access (TDMA) and employing atiming advance to shift the time of transmission of a mobile radiostation relative to the time instant set by a sync signal transmittedfrom the base station in such a way that the timing advance compensatesfor the transmission delay caused by the distance between the basestation and the mobile station, and the transmission of the mobilestation is received at the base station in the correct TDMA time slot.The invention is especially suitable for use in the GSM and DCS1800mobile radio systems. The GSM specifications and The GSM System forMobile Communications (by M. Mouly and M.- B. Pautet, Palaiseau, France,1992, ISBN:2-9507190-0-7) are referred to for a more detaileddescription of the GSM system.

FIG. 1 shows a base station according to the invention. Referring to thebase station, only parts and functions essential to the understanding ofthe invention will be described, i.e. mainly the timing of thetransceivers of the base station. In addition, in the example, a TDMAsystem where communication takes place in frames of 8 time slots on aradio channel will be described.

In FIG. 1, a radio transmitter T×1 and a radio receiver form atransceiver fully similar to a conventional base station transceiver instructure and operation. At the base station a time slot clock generator25 and a frame clock generator 26 generate a time slot clock 25A and aframe clock 26A, respectively, required for the transceiver. Referringto FIG. 2, the TDMA frame period of the transceiver T×1/R×1 starts at atime to defined by the frame clock pulse 26A, and the first time slot ofthe frame begins at the same time. The following time slots of the framebegin at times t₁ -t₇ determined by the time slot clock pulses 25A,until a new frame begins at a time t₀ determined by a new frame clockpulse 26A. In this specific case, one frame thus contains 8 time slots,but the number of time slots may be smaller or greater, e.g. 4,depending on the system. In FIG. 2, the time slots are numbered from TS0to TS7. In the transceiver T×1/R×1, operating in a normal way, the frameperiods of both the receiver and the transmitter begin at the same timet₀. Correspondingly, the time slots begin at the same times t₀ -t₇. Timeslot numberings, however, differ in that each time slot number occurs onthe reception side three time slots later than the respective time slotnumber on the transmission side.

The operation of the transceiver T×1/R×1 will be discussed withreference to FIGS. 1 and 2 while assuming that a mobile radio stationMS1 and the transceiver T×1/R×1 communicate in the TDMA time slot 2.Under the control of the clock signals 25A and 26A, the transmitter T×1starts the transmission of a burst addressed to the mobile station MS1after the time t₂. The assembled radio-frequency burst is transmittedvia a transmitting antenna 30 to the mobile station MS1 on a carrier F1.The mobile station MS1 receives the burst, is synchronized with the basestation on the basis of synchronizing information contained in theburst, and transmits the radio-frequency burst to the base station on acarrier F2 approximately 3 time slots later. The receiver R×1 receivesthe burst via a receiving antenna 31 in the reception time slot 2between t₅ and t₆. Successful reception requires that the burst receivedfrom the mobile station MS1 fall within the right time slot at thereceiver R×1. However, if the mobile station MS1 is remote from the basestation, propagation delays caused by the distance may delay the burstto such an extent that bursts from mobile stations MS using adjacenttime slots overlap. For this reason, the transceiver T×1/R×1 measuresthe time difference occurring between the transmissions of thetransceiver and the mobile station due to the distance between the basestation and the mobile station. On the basis of the measurement, thetransceiver calculates a timing advance required for the mobile stationfor compensating for the propagation delay caused by the distance. Thebase station signals this timing advance information to the mobilestation MS1, which advances its time of transmission from the timedetermined by the burst received from the base station by an amountcorresponding to the timing advance. In this way, the burst transmittedby the mobile station will reach the base station receiver R×1 at theright time. Various intra-system limitations, however, set a maximumvalue AD_(MAX) for the timing advance, and this maximum value in turndetermines the maximum distance at which the propagation delay can stillbe compensated for. Accordingly, the maximum of the timing advance haslimited the cell size of conventional base stations.

For this purpose, the base station shown in FIG. 1 comprises anothertransceiver having a transmitter T×2 and a receiver R×2, whereby thetiming of the receiver R×2 is delayed with respect to the transmitterT×2. This is accomplished by applying the time slot clock signal 25A andthe frame clock signal 26A generated by the clock generators 25 and 26directly to the transmitter T×2 but through a delay means 27 to thereceiver R×2. The delay means 27 produces a delayed frame clock signal27A and a delayed time slot clock signal 27B.

The operation of the transceiver according to the present invention willagain be described with reference to FIGS. 1 and 2 while assuming thatcommunication between the base station and the mobile station MS2 takesplace in the time slot 2. The transceiver T×2 transmits a burst to themobile station MS2 under the guidance of the frame and time slot signals25A and 26A in the time slot 2 between t₂ and t₃. The radio-frequencyburst is transmitted via the transmitting antenna 32 on a radio channelF3 to the mobile station MS2. The mobile station MS2 receives the burst,is synchronized with the base station on the basis of the synchronizinginformation contained in the burst, and transmits the burst to the basestation on a radio channel F4 about three time slots later, depending onthe timing advance. So far, the operation is similar to that of thetransceiver T×1/R×1.

Now, however, the distance of the mobile station MS2 from the basestation is greater than the maximum distance determined by the maximumvalue AD_(MAX) of the timing advance. The timing advance is thereforenot able to compensate for the propagation delay caused by the distance,and a burst transmitted by the mobile station MS2 would arrive at aconventional receiver partly during the following time slot. Accordingto the invention, the time of reception of the receiver R×2 is delayedwith respect to the timing of the transmitter T×2 by a predetermineddelay dT. Referring to FIG. 2, the frame clock pulse arrives at thetransmitter T×2 and starts a new frame at t₀. However, the correspondingframe clock pulse, delayed by the delay means 27, does not arrive at thereceiver R×2 until at the time t'₀ =t₀ +dT. Similarly, each time slotpulse arrives at the receiver R×2 delayed by the delay dT relative tothe transmitter T×2. In this way, the burst from the mobile station MS2will have a period corresponding to the delay dT more time to arrive atthe base station and fall into the right time slot. This, in turn, meansthat the distance of the mobile station MS2 from the base station can beincreased by an amount corresponding to the propagation delay dT, i.e.the cell size of the base station increases. On the other hand, thereceiver R×2 is not able to receive bursts from mobile stations thedistance of which from the base station is shorter than the propagationdelay dT, as bursts from these mobile stations arrive at the basestation too early. In this way, the transceiver T×2/R×2 will have aring-shaped service area, the inner radius r₁ of which is directlyproportional to the delay dT and the outer radius r₂ of which is r₁+r_(max), where r_(max) is a maximum distance corresponding to themaximum value AD_(MAX) of the timing advance. The required receptiondelay dT can be calculated from the equation

    dT=r.sub.1 /C,

where C is the velocity of light. To obtain a cell size extension of 20km, for instance, the delay value will be about 67 μs.

FIG. 3 shows the service area of a conventional transceiver T×1/R×1, themaximum radius r_(MAX) of which is determined by the timing advance, andthe service area of a transceiver R×2/T×2 according to the invention,the maximum radius of which is r₂ =r₁ +r_(MAX). If both transceivertypes are used at the same base station, the cell size of conventionalbase stations will be increased significantly.

In the cell shown in FIG. 3, the service areas of the conventionaltransceiver and the transceiver according to the invention preferablyoverlap to some extent, which allows interference-free handover withinthe base station from one transceiver to the other. Various criteria maybe applied in handover, including the value of the timing advanceassigned to the mobile station MS. For instance, when a mobile stationlocated within the service area of the transceiver according to theinvention moves in the direction indicated by the arrow A1 to theservice area of the conventional transmitter, the base station performsa forced internal handover when the timing advance assigned to themobile station falls below a predetermined value. Correspondingly, whena mobile station moves from the service area of the conventionaltransceiver to the service area of the transceiver according to theinvention, the base station performs a forced internal handover when thetiming advance assigned to the mobile station exceeds a predeterminedvalue. The handover can be controlled from a base station controller 28.The base station controller 28 may also vary the delay dT set by thedelay means 27 by a control signal 28A, e.g. within the range from 1 to116 μs. The variable delay dT allows the location of the ring-shapedservice area of the transceiver according to the invention to be alteredin accordance with the requirements of the radio network, e.g. its load.

In certain special cases it may be possible to use a base stationcomprising merely transceivers according to the invention, which areprovided with a delayed reception. The base station thereby has aring-shaped cell where no services are provided for subscribers in theimmediate vicinity of the base station. One such special case is shownin FIG. 4, where two transceivers according to the invention areconnected to different directional antennas so that they will haveseparate elongated services areas 41 and 42 directed in differentdirections from the base station BTS. The base station BTS may bepositioned at a high point in the terrain, in a mast, or in a buildingremote from a road 44 passing by so that there is no need for serviceprovision in the immediate vicinity of the station, and a shadow area 43will not be disadvantageous. However, the extended service areas 41, 42of the base station cover a very long section of the road passing by,while several base stations have previously been needed for the roadsection. Intracell handover is here performed by a normal handoverprocedure.

The figures and the description related to them are only intended toillustrate the present invention. In its details, the base stationaccording to the invention may vary within the scope of the attachedclaims.

I claim:
 1. A base station for a digital time division multiple access(TDMA) radio network, the base station comprising:a radio transceiver,means for measuring a time difference between the transmissions of thetransceiver and a mobile radio station due to a distance between thebase station and the mobile radio station, means for controlling themobile radio station to adjust its transmission relative to thetransmission of the transceiver by a predetermined timing advancecompensating for said time difference, said timing advance having amaximum value determining a maximum radius r_(MAX) for an effectiveservice area of the base station transceiver, means for providing atiming offset between the reception and the transmission of thetransceiver in a manner that the maximum radius of the service area ofthe transceiver is r₁ +r_(MAX), and the service area has a central areawhich has a radius r₁ and within which the transceiver is not capable ofproviding services to mobile radio stations, where r₁ is directlyproportional to said timing offset, and a second radio transceiver wherethe timing between reception and transmission is normal and the outerradius of the service area of which is at a distance r_(MAX) from thebase station, said offset between transmission and reception beingselected so that the service areas of the first-mentioned transceiverand the second transceiver overlap.
 2. A base station according to claim1, wherein:said timing offset dT between the reception and transmissionis calculated from the equation

    dT=r.sub.1 /C,

where C is the velocity of light.
 3. The base station according to claim1, wherein:said timing offset between the reception and transmission isvariable.
 4. The base station according to claim 1, wherein the servicearea of the first-mentioned transceiver is ring-shaped.
 5. The basestation according to claim 1, wherein:the first-mentioned transceiverhas a directional antenna and an elongated service area in a certaindirection from the base station.
 6. The base station according to claim5, wherein:the base station comprises another transceiver where thetiming of reception is delayed relative to transmission by apredetermined delay, and that said first-mentioned transceiver has adirectional antenna and an elongated service area in a direction awayfrom the base station, which direction is different from the directionof the service area of the first-mentioned transceiver.
 7. The basestation according to claim 6, wherein:service areas of the transceiverscover a long road section.
 8. The base station according to claim 1,including:means for performing the internal handover of the basestation, when a mobile radio station moves from one service area of thebase station to the other service area.
 9. The base station according toclaim 8, wherein:said handover means perform the handover on the basisof the timing advance of the mobile radio station.
 10. The base stationaccording to claim 9, wherein:said handover means perform a handoverwhen the timing advance of a mobile radio station located within theservice area of the second transceiver exceeds a predetermined value.11. The base station according to claim 8, wherein:said handover meansperform a handover when the timing advance of a mobile radio stationlocated within the service area of the first-mentioned transceiver fallsbelow a predetermined value.